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United States Patent |
6,190,625
|
Jha
,   et al.
|
February 20, 2001
|
Fluidized-bed roasting of molybdenite concentrates
Abstract
The present invention provides a method for fluidized bed roasting of
molybdenite to molybdenum trioxide. A fluid bed reactor separated into
separate zones is used to provide plug flow conditions. A cooling tube is
submerged in the fluid bed to control temperature. A vibrator is used to
enhance fluidization.
Inventors:
|
Jha; Mahesh C. (Golden, CO);
May; William A. (Boulder, CO)
|
Assignee:
|
Qualchem, Inc. (Golden, CO)
|
Appl. No.:
|
907373 |
Filed:
|
August 7, 1997 |
Current U.S. Class: |
423/53; 266/172; 423/659 |
Intern'l Class: |
C01G 039/02; C22B 001/10 |
Field of Search: |
266/172,175,176
422/146
423/53,606,659 F
|
References Cited
U.S. Patent Documents
2371619 | Mar., 1945 | Hartley | 266/172.
|
2404944 | Jul., 1946 | Brassert | 266/172.
|
2637629 | May., 1953 | Lewis | 423/542.
|
2756986 | Jul., 1956 | Schytil et al. | 266/159.
|
3455677 | Jul., 1969 | Litz | 75/419.
|
3656933 | Apr., 1972 | Wolf et al. | 423/148.
|
3745668 | Jul., 1973 | Vian-Ortuno et al. | 34/589.
|
3798306 | Mar., 1974 | Lapat et al. | 423/50.
|
3897546 | Jul., 1975 | Beranek et al. | 423/659.
|
3941867 | Mar., 1976 | Wilkomirsky et al. | 423/53.
|
4409101 | Oct., 1983 | Salikhov et al. | 210/266.
|
4626279 | Dec., 1986 | Bjornberg et al. | 423/22.
|
5133137 | Jul., 1992 | Petersen | 34/576.
|
5876679 | Mar., 1999 | D'Acierno et al. | 422/143.
|
Foreign Patent Documents |
0 274 187 A2 | Oct., 1987 | EP.
| |
Other References
Wilkomirski, I.A., A.P. Watkinson, J.K. Brimacombe, Recirculating
fluidized-bed process for the roasting of molydbenite concentrates,
Institute of Mining and Metallury; Dec. 1975, pp. C197-c205.
Roasting of Molydbenite, pp. 316-326.
Zelikman, A.N., L.V. Belyaevskaya, G.M. Vol'dman, and T.E. Prosenkova;
Fluidized Bed Roasting of Granulated Molybdenite Concentrate; pp. 65-70.
Lindsay, D.G.; Endako Roasting Practice; The Metallurgical Society of CIM;
Annual vol. 1977; pp. 32-36.
|
Primary Examiner: Beck; Shrive
Assistant Examiner: Varcoe; Frederick
Attorney, Agent or Firm: Marsh, Fischmann & Breyfogle LLP
Claims
What is claimed is:
1. A method for converting molybdenum sulfides into molybdenum oxides,
comprising:
introducing a feed material comprising molybdenum sulfides into a fluidized
bed of particulate solids contained within a fluidized bed reactor, the
fluidized bed being separated into a plurality of different zones with
adjacent zones being separated by a baffle with an opening between the
adjacent zones such that the material flows through the fluidized bed
reactor in plug flow mode;
contacting the particulate solids of the fluidized bed, including the feed
material, with an oxygen containing fluidizing gas while vibrating the
fluidized bed reactor, to convert at least a portion of the molybdenum
sulfides into molybdenum oxides and thereby form a product comprising
mnolybdenum oxides;
maintaining a fluidized bed temperature in a range of from about
400.degree. C. to about 580.degree. C., comprising (i) cooling at least a
portion of the particulate solids in at least a first zone of the
fluidized bed, during the contacting step, with cooling means for removing
heat from the first zone of the fluidized bed to maintain a fluidized bed
temperature in the first zone of no more than about 580.degree. C., the
cooling means being submerged in the fluidized bed during the cooling
step, an cd (ii) heating at least a portion of the particulate solids in
at least a second zone of the fluidized bed to maintain a fluidized bed
temperature in the second zone of at least about 400.degree. C.;
wherein, the particulate solids in the first zone have a higher sulfide
content, than the sulfide content of the particulate solids in the second
zone, and exothermic reaction of the sulfides in the first zone produces
excess heat in the first zone and heat is removed from the first zone by
the cooling to maintain the temperature in the first zone within the
range;
wherein, the particulate solids of the second zone have a lower sulfide
content, than the sulfide content of particulate solids in the first zone,
and exothermic reaction of sulfides in the second zone produces
insufficient beat in the second zone and heat is added to the second zone
by the heating to maintain the temperature in the second zone within the
the range; and
removing product comprising the molybdenum oxides from the fluidized bed
reactor.
2. The method of claim 1, wherein the feed material has a P.sub.80 size of
no more than about 100 mesh (Tyler).
3. The method of claim 1, wherein the feed material has a molybdenum
sulfide concentration of at least about 50 wt %.
4. The method of claim 1, wherein at least about 95% of the molybdenum
sulfides in the feed material are converted into molybdenum oxides during
the contacting step.
5. The method of claim 4, wherein the method is conducted substantially in
the absence of recycle of the particulate solids.
6. The method of claim 1, wherein the product comprises no more than about
5 wt % molybdenum sulfides.
7. The method of claim 1, wherein the product is in the form of a
free-flowing particulate material.
8. The method of claim 1, wherein the amplitude of vibration of the
fluidized bed reactor ranges from about 1/16 inch to about 1/2 inch.
9. The method of claim 1, wherein the fluidizing gas comprises at least
about 10 vol % oxygen.
10. The method of claim 1, wherein the contacting step comprises:
contacting a first portion of the feed material with a first fluidizing gas
in the first zone of the fluidized bed to convert a first portion of the
molybdenum sulfides to molybdenum oxides; and
contacting a second portion of the feed material with a second fluidizing
gas in the second zone of the fluidized bed to convert a second portion of
the molybdenum sulfides to molybdenum oxides.
11. The method of claim 10, wherein the first and second zones are
separated by a baffle with a relatively small opening such that the
material flows from the first zone to the second zone in a plug flow mode.
12. The method of claim 10, wherein the oxygen content of the first
fluidizing gas is greater than the oxygen content of the second fluidizing
gas.
13. The method of claim 10, wherein the flow rate of the first fluidizing
gas is greater than the flow rate of the second fluidizing gas.
14. The method of claim 10, wherein the fluidized bed depth in the first
zone is less than the fluidized bed depth in the second zone and the
residence time in the first zone is less than the residence time in the
second zone.
15. The method of claim 1, wherein the feed material is comprised
substantially entirely of molybdenite concentrate.
16. The method of claim 1, wherein at least a portion of the fluidizing gas
is preheated prior to introduction into the fluidized bed.
17. The method of claim 1, wherein the residence time of the particulate
solids in the fluidized bed is no longer than about 8 hours.
18. The method of claim 1, wherein the fluidized bed has a top and a bottom
and the baffle extends from the top of the fluidized bed to the bottom of
the fluidized bed.
19. The method of claim 1, wherein the fluidized bed is located in a lower
section of the fluidized bed reactor, the fluidized bed reactor further
comprising an upper section located above the fluidized bed, the upper
section having a larger width than the lower section, so that the velocity
of the fluidizing gas decreases as the fluidizing gas flows from the lower
section into the upper section.
20. The method of claim 19, wherein the upper section is in fluid
communication with at least one separation device which removes entrained
particles from the upper section and returns the entrained particles to
the fluidized bed.
21. The method of claim 1, wherein the cooling means comprises a cooling
tube submerged in the fluidized bed, the cooling comprising passing a
cooling fluid through the cooling tube to remove heat from the fluidized
bed.
22. The method of claim 1, wherein the first zone and the second zone are
in fluid communication via a space in the fluid bed reactor located above
the top of the baffle.
23. The method of claim 1, wherein the fluidized bed includes at least
three zones with the second zone being the last in series of the at least
three zones.
24. The method of claim 23, wherein delivery of the fluidizing gas to each
of the plurality of zones is controlled separately, and the fluidizing gas
delivered to at least the second zone is preheated prior to introduction
into the second zone.
Description
FIELD OF THE INVENTION
The present invention is directed generally to fluidized-bed roasting and
specifically to fluidized-bed roasting of molybdenite concentrates.
BACKGROUND OF THE INVENTION
Molybdenum oxide, particularly MoO.sub.3, is widely used as a raw material
in the manufacture of stainless and low alloy steels, pure molybdenum
metal, superalloys, catalysts, and specialty chemicals. Molybdenum oxide
is commonly produced from molybdenum sulfides, particularly molybdenum
concentrates, that are obtained by grinding copper or molybdenum ores and
concentrating the sulfides contained therein.
To convert molybdenum sulfide to molybdenum oxide, molybdenum sulfide is
typically air roasted in a multiple-hearth roaster according to the
following overall reaction:
MoS.sub.2 +3.50.sub.2.fwdarw.MoO.sub.3 +2SO.sub.2.
In multiple-hearth roasters, a static bed of feed material containing the
molybdenum sulfides is calcined by an oxidizing gas (air) at temperatures
ranging from 550 to over 700.degree. C. To avoid severe fusion of the bed,
operators periodically mix the bed manually to maintain bed porosity and
permeability at desired levels.
In designing a more efficient roaster for molybdenum sulfide concentrates,
there are a number of important considerations. By way of example, the
roaster should be continuous and produce a uniform quality, low-sulfur
molybdenum oxide product. To reduce the capital and operating costs of gas
handling and acid plants, it is desirable to minimize the use of excess
air and thereby produce a roaster off-gas containing a relatively high
concentration of SO.sub.2. Second, the roaster should be capable of
automated operation to provide reduced operating costs. In other words,
the roaster design should allow automated process control at operating
conditions that will allow long operating times without downtime for
cleaning and maintenance. Third, the roaster should recover heat energy
released during sulfide oxidation as steam for useful purposes. Fourth,
the roaster should have relatively few moving parts to improve system
reliability, simplify system operation and decrease downtime and
maintenance costs. Fifth, the roaster should provide for substantially
uniform distribution of heat of reaction throughout the bed. The existence
of temperature gradients in the bed can create hot zones where high
temperature can cause partial fusion and sintering of the bed and
volatilization of molybdenum oxide. Sixth, the roaster should have little,
if any, refractory lining. Refractory lining can cause molybdenum loss
(via molybdenum penetration into the refractory lining) and product
contamination. Seventh, the roaster should eliminate the formation of hard
crusts of molybdates and oxides. Such crusts can erode moving parts (e.g.,
rabble arms and teeth in conventional multiple-hearth roasters) and
increase operating costs through increased labor to clean the roaster.
Eighth, the roaster off-gas should have little, if any, dust entrainment
to minimize product loss and downstream gas cleaning costs. Ninth, the
roaster should be capable of handling feed materials comprising
impurities, such as calcium, copper, iron and rhenium without operational
problems. Finally, the roaster should operate at a temperature low enough
to retard the volatilization of molybdenum trioxide and later condensation
of the molybdenum trioxide in cooler pipes. Such condensation can cause
operational and maintenance problems and reductions in product yields.
SUMMARY OF THE INVENTION
It is an objective of the present invention to provide a continuous
roasting apparatus and method that can produce a uniform quality
molybdenum oxide product at relatively low operating and capital costs.
Related objectives include providing a roasting apparatus that is amenable
to automatic process control; recovers energy released during sulfide
oxidation; has few moving parts; has no refractory lining; can eliminate
the formation of hard crusts of sintered molybdates and oxides, minimizes
dust entrainment in the roaster off-gas, can handle feed materials
comprising impurities without operation problems and can retard the
volatilization of molybdenum trioxide.
These and other objectives are addressed by the present invention which
provides a fluidized bed, roasting apparatus including:
(a) a chamber (e.g., a fully enclosed housing) for containing a bed of a
feed material including molybdenum sulfides, such as molybdenite
concentrates, the chamber having a feed port for introducing the feed
material including molybdenum sulfides into the bed and a discharge port
for removing the product from the bed;
(b) fluidizing means (e.g., a blower and connected ductwork) for contacting
a fluidizing gas such as air with the bed to convert the molybdenum
sulfides into molybdenum oxides, such as MoO.sub.3, and sulfur oxides,
such as SO.sub.2.
(c) cooling means (e.g., a cooling tube containing a cooling fluid such as
water) for removing the heat generated by the roasting reaction and
thereby maintaining the bed at a desired temperature.
Because the conversion of molybdenum sulfides to molybdenum oxides is
strongly exothermic, the cooling means closely controls the fluidized bed
temperature. The cooling means can include a heat exchange fluid, such as
water/steam, for recovering energy released during sulfide oxidation. The
cooling means enables precise control of temperature at a set point which
retards fusion and/or sintering of the materials in the fluidized bed,
eliminates the need for a refractory lining, and retards volatilization of
molybdenum trioxide. Preferably, the cooling means maintains the fluidized
bed temperature below the sublimation and melting points of the molybdenum
oxide and molybdate compounds that can form due to the presence of
impurities. More preferably, the maximum fluidized bed temperature is less
than about 580.degree. C.
The roasting apparatus of the present invention has numerous advantages
relative to conventional multiple hearth roasters. By way of example, the
roasting apparatus of the present invention can be continuous, have a high
throughput and relatively low capital and operating costs. The roasting
apparatus can be amenable to automated process control, can handle feed
materials comprising impurities such as calcium, copper, iron and rhenium
without operation problems, and can have relatively few moving parts and
therefore a high degree of system reliability and simplified system
operation.
To eliminate gas channeling which causes pockets of unfluidized solids in
the bed and to provide for a relatively high degree of fluidized bed
porosity and permeability, the roasting apparatus can include vibratory
means (e.g., a vibrator) for vibrating the bed during roasting.
Preferably, the minimum amplitude of vibration of the vibratory means is
about 1/16" and the maximum amplitude is about 1/2"
To inhibit entrainment of finely-sized feed material in the roaster
off-gas, the chamber can have an expanded section at its upper end to
provide for decreased velocity of the roaster off-gas. Entrained particles
will return to the bed in response to the decrease in fluidizing gas
velocity from the increased area of flow and decreasing gas temperatures
due to heat losses from the upper roaster walls. Additionally, the
apparatus can include gas/particulate separator means (e.g., a cyclone) to
separate any remaining entrained particles from the roaster off-gas.
To provide for substantially uniform fluidizing gas (e.g., air)
distribution and heat of reaction throughout the bed, the roasting
apparatus can include a distributor plate located below the fluidized bed
for distributing the fluidizing gas very uniformly across the bottom
portion of the bed. Preferably the distribution plate has a plurality of
pores for passage of the fluidizing gas, with the maximum distance between
the centers of adjacent pores being about 1/4 inch.
To provide for plug flow of the feed material as the material moves through
the roasting apparatus, the apparatus can include one or more baffles to
divide the chamber into a number of reaction zones. The baffle(s) extend
above the distributor plate and have an opening which allows the feed
material to move by plug flow from an upstream zone to an adjacent
downstream zone. As will be appreciated, plug flow conditions ensure that
each particle in the bed will have substantially the same residence time
in each zone and therefore the same degree of conversion as it exits the
roaster.
To provide for differing residence times in different reaction zones, the
distributor plate and/or bottom member of the chamber can slope downwardly
from the feed port to the discharge port. The sloping distributor plate
provides for a bed of varying depths from zone to zone with the upstream
zone(s) having a lesser bed depth (and therefore a lower residence time)
than the downstream zone(s).
Because the percentage of unroasted sulfide decreases in each successive
downstream zone, the quantity of oxygen required and exothermic heat
generated also decreases. Therefore, the apparatus can include control
means for independently controlling the fluidized bed temperature and
fluidizing gas flow rate in each of the zones. Accordingly, each of the
zones can have separate heaters, cooling means, and flow controllers to
provide for differing fluidized bed operating conditions.
To further control fluidized bed temperature, the roasting apparatus can
include a means for preheating the fluidizing gas before contacting of the
fluidizing gas with the bed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a fluidized bed roaster according to
the present invention taken along line 1--1 of FIG. 2;
FIG. 2 is a cross-sectional view of the fluidized bed roaster taken along
line 2--2 of FIG. 1;
FIG. 3 is a cross-sectional view of another embodiment of a fluidized bed
roaster according to the present invention taken along line 3--3 of FIG.
4; and
FIG. 4 is a cross-sectional view of the fluidized bed roaster taken along
line 4--4 of FIG. 3.
DETAILED DESCRIPTION
The present invention is directed to a fluidized-bed roasting apparatus and
method for converting sulfides to oxides at high conversion rates.
Although the roasting apparatus can effectively oxidize any sulfide ore or
concentrate, the process is particularly effective in roasting molybdenum
sulfide containing feed materials such as molybdenite concentrates
produced from copper or molybdenum ores and chemically produced MoS.sub.2
or MoS.sub.3 precipitates and mixtures thereof. They may contain
impurities such as calcium, copper and iron. The molybdenum oxides
produced by roasting can include molybdenum trioxide (MoO.sub.3) and
molybdenum dioxide (MoO.sub.2), and mixtures thereof. Depending upon the
feed material, the product may have a high enough purity to be sold as
"Tech Oxide" or it can be converted to high-purity molybdenum chemicals.
An embodiment of the fluidized-bed roasting apparatus of the present
invention is depicted in FIGS. 1-2. As will be appreciated, the actual
dimensions and operating conditions of the roasting apparatus in FIGS. 1-2
can vary depending upon the desired throughput rate and feed
characteristics.
The roasting apparatus or roaster 10 includes a rectangular chamber 14
having a feed port 18 for the feed material, a discharge port 22 for the
product, vertical baffles 30a,b for dividing the chamber interior into
multiple zones 34a-c, and a distributor plate 42. A fluidized bed 46 of
particulate solids (feed material, product material and mixtures thereof)
is located in the chamber 14 above the distributor plate 42. A plenum
chamber 54 for distribution of the fluidizing gas is located below the
distributor plate 42. The material moves from the feed end to the
discharge end of the roaster through openings 58a,b in the baffles in a
plug flow mode.
The interior of the chamber 14 is sealed from the ambient atmosphere to
permit subatmospheric pressure operation and to inhibit release of
potentially harmful byproduct gases into the atmosphere.
The feed port 18 may include a volumetric feeder or other similar device
for providing a controlled feed rate of the feed material continuously to
the portion of the fluidized bed 46 of material contained within the first
zone 34a of the chamber 14.
The discharge port 22 may include a rotary valve or other similar device
for discharging the hot calcined product from the last zone 34c of the
chamber 14.
Referring to the end view shown in FIG. 2 to retard entrainment of finely
sized particles in the roaster off-gas 50, the upper section 62 of the
roaster chamber 14 is expanded to increase the internal cross-sectional
area. As will be appreciated, molybdenite concentrates contain a wide
particle size distribution from about 100 microns to 1 micron or less. The
expanded upper section 62 provides for gradual reductions in the gas
velocity to remove such finely-sized particles from the roaster off-gas
50.
Referring again to FIGS. 1 and 2 and as noted above to control operating
conditions in different areas of the roaster, the interior of the roaster
chamber 14 is divided into a plurality of zones 34a-c. The reaction
conditions in each zone can be different due to differing sulfide contents
of the portions of the bed 46 in each zone.
Each of the adjacent zones 34a-c is substantially isolated from adjacent
zones to permit the reaction conditions in each zone to be independently
monitored and controlled. Zone isolation is important to optimize the
reaction conditions in each zone. To provide zone isolation, the upper
portion of each of the baffles 30a,b extends from the distributor plate 42
to a point near a top panel 66 while a lower portion extends from the
bottom of the distributor plate 42 to the bottom of the plenum chamber 54.
Small openings 58a,b in the baffles allow movement of the material from
one zone to the next in a plug flow mode. In this manner, the plenum
chamber 54 is divided into separate sections, with each section
corresponding to a zone. Although the optimum number of zones depends upon
the characteristics of the feed material, it is generally preferable to
have from about 3 to about 12 zones in a fluidized roasting reactor.
The distributor plate 42 has a relatively fine and tightly spaced hole
pattern to provide for a fine dispersion of the fluidizing gas along the
length of the bed 46 and therefore a substantially uniform distribution of
the fluidizing gas 70a-c across the bottom of the bed 46 in each
respective zone. The distributor plate 42 has a plurality of perforations
or pores, with the maximum spacing between the centers of the perforations
or pores being about 0.25 inches.
The finely dispersed and substantially uniform distribution of the
fluidizing gas 70 along the length of the distributor plate 42 is
important to maintaining bed porosity and permeability and controlling bed
temperature. Molybdenum trioxide can react with other metal oxides
(impurities) to form low melting point molybdates, which can aggregate and
sinter the bed at temperatures even below 580.degree. C. To complicate
matters even further, oxidation of molybdenite is a highly exothermic
reaction. A highly efficient distributor plate 42 overcomes this problem
by distributing the fluidizing gas uniformly throughout the bed thereby
eliminating pockets of unfluidized material from which the heat of
reaction cannot be removed. Such pockets result in localized overheating
leading to agglomeration and defluidization of the bed.
To provide temperature control, the upstream zones 34a,b include a cooling
tube 78a,b for selective heat removal in the portion of the bed in each
chamber 34a,b. The cooling tubes 78a,b are submerged in the bed 46 and
configured such that the temperature of the bed in a specific zone is
controlled by the respective tube 78a,b. It is important to provide the
cooling tube surface substantially uniformly throughout the reaction zone.
The shape of the cooling tube and its size depend, of course, upon the
design and size of the individual zones. Thus, as shown in FIG. 1, each of
the cooling tubes 78 can be a dip tube (or a cooling coil) extending from
the top of the corresponding portion of the bed to the bottom.
Each of the cooling tubes 78a,b can be collectively connected to a single
cooling fluid input line 82 for supplying a metered amount of cooling
fluid to each of the tubes. Although many fluids can be used as a cooling
fluid, the cooling fluid is preferably water, which is converted into
steam when passing through the tube in response to the heat of reaction.
The steam is removed via a common header 86 for other activities in the
plant or for sale, as desired.
Alternatively, the tubes 78a,b can be connected to separate cooling fluid
input lines to permit the supply of differing amounts of the cooling fluid
to different zones. In this manner, the temperature in each of the zones
can be independently controlled in response to different reaction
conditions in each of the zones. This method of cooling the bed
temperature is easily adapted to automatic computer control. Cooling tubes
are unnecessary in the final zone 34cof roasting when very little heat is
generated during the final stages of sulfur removal and molybdenum
oxidation.
The temperature of the various zones of the fluidized bed is, of course, an
important operational parameter. Molybdenite roasting is initiated at
temperatures as low as 400.degree. C., but the rate of roasting is still
slow at 500.degree. C. and increases rapidly as the temperature is
increased above this point. Thus the temperature in the fluidized bed is
maintained between about 500 and about 580.degree. C. The optimum roasting
temperature for each zone is dependent upon the feed composition.
Depending upon the feed material composition, the zones can have different
temperature profiles. The operating temperature in each zone can be
independently controlled to substantially optimize process conditions for
the bed material in that zone. Typically, heat must be added to the final
zone 34c by preheating the fluidizing gas and if required heating the
external walls of the reactor to maintain the desired temperature.
To heat the reactor walls, the final zone 34c can have separate heaters
90a,b (shown in FIG. 2) positioned adjacent to the zone to maintain the
fluidized bed temperature at desired levels. Preferably, the heaters are
separately controlled to reflect the differing heat requirements in the
downstream zones. As will be appreciated, for some feed materials
additional zones may require additional heat input via a heater.
At the recommended temperatures of operation, no more than about
580.degree. C., it is feasible to use stainless steel for construction of
the roaster without a refractory lining. This not only reduces the size
and cost of the roaster, but allows for quick shutdown and startup of the
roaster. Operation under these conditions also eliminates the problem of
molybdenum lost to refractories, contamination of the product with
refractories, and heavy maintenance required to repair the brick walls.
Because of the fine sizes, cohesive nature, and high angle of repose of the
particles in the bed 46, it is necessary to vibrate the chamber 14 during
fluidization to maintain desired levels of bed porosity and permeability
and therefore eliminate sintering/fusion of the bed 46. Vibration of the
chamber 14 can eliminate channeling (i.e., short circuiting) of fluidizing
gas in the bed, knock off any solids build-up on the reactor walls and
cooling tubes, and improve heat transfer. This improves bed fluidization
and eliminates pockets of unfluidized material. For this reason, a
vibrator 94 is connected externally to the chamber 14, and the entire
chamber 14 is mounted on a number of flexible supports (not shown), such
as springs, to provide the chamber 14 and attached components with freedom
of motion. The vibrator 94 moves the entire chamber 14 back and forth in a
substantially horizontal plane during fluidization. Although the design
and size of the vibrator depend on the size of the roaster (which in turn
depends upon the throughput rate), the vibrator is selected to provide a
minimum amplitude of vibration of the chamber 14 of about 1/16" and a
maximum amplitude of about 1/2".
The roaster 10 further includes a fluidizing gas handling system for
fluidizing the bed 46. The fluidizing gas handling system includes one or
more blowers/compressors (not shown), gas metering flowmeter (not shown),
fluidizing gas preheaters 98a-c, and gas input and output lines. The
fluidizing gas 70 preferably has an oxygen content of at least about 15
vol % with the preferred fluidizing gas being air. The preheaters 98a-c
preferably heat the fluidizing gas 70 to a minimum temperature of about
300.degree. C. and a maximum temperature of about 700.degree. C. A
separate preheater and flowmeter device is located upstream of each zone
section of the plenum chamber 54. Preferred preheaters are electric or gas
fired to permit precise control of the fludizing gas temperature. The
preheaters are used during startup as well as to maintain the bed
temperature, especially in the final zone 34C.
The off-gas 50 from the roaster is passed through a gas/particulate
separator device 110 to form a treated off-gas 52. The gas/particulate
separator device 110 can be any suitable device for removing entrained
particulates from gases, with cyclones being most preferred. The removed
particles are returned to the bed 46 and the treated off-gas 52 is
conveyed to a gas scrubber. If necessary, an additional gas/particulate
separator device can be used to further treat the off-gas to reduce
further the entrainment of dust particles. These recovered particles are
also returned to the bed 46.
The operation of the roaster 10 can be automated by a monitoring and
control system. The monitoring and control system (not shown) can include
thermocouples positioned at various locations in each zone to measure
temperature, flowmeters in the fluidizing gas input lines to each zone to
measure fluidizing gas flow rate, thermocouples positioned in the
fluidizing gas input lines to each zone to measure fluidizing gas
temperature prior to contacting of the bed, flowcontrollers positioned in
the cooling fluid input lines for controlling flow rate, a feed controller
for controlling feed material feedrate, and a control device, e.g., a
central processor, for adjusting one or more of the fluidizing gas
temperature or flow rate, cooling fluid flow rate, feed rate of the feed
material, fluidized bed temperature to substantially optimize and control
processing conditions.
The operation of the roasting apparatus will be described with reference to
FIGS. 1-2. Although the feed material can be in any form, the description
will be based upon the conversion of a concentrated feed material derived
from mining a naturally occurring deposit.
As a primary concentrate formed from molybdenum ores or a byproduct
concentrate formed from ores of other metals such as copper, the feed
material can contain a variety of compounds. By way of example, the feed
material can contain molybdenum sulfides, copper sulfides, rhenium
sulfides, and iron sulfide. Preferably, the feed material has a molybdenum
sulfide content ranging from about 50 to about 100 wt %, more preferably
from about 75 to about 98 wt %, and most preferably from about 90 to about
95 wt %. In most applications, the feed material has a relatively fine
particle size distribution. Preferably, the P.sub.80 size of the feed
material is no more than about 100 mesh (Tyler) and more preferably no
more than about 150 mesh (Tyler) and most preferably no more than about
200 mesh (Tyler).
The feed material is supplied to the bed in the first zone 34 a via feed
port 18. Fluidizing gas, preferably air, is preheated and supplied to the
various sections of the plenum chamber 54. The fluidizing gas comprises
preferably at least about 10 vol % molecular oxygen and more preferably at
least about 20 vol % molecular oxygen. The fluidizing gas passes through
the distributor plate 42 and oxidizes molybdenum sulfides and other
sulfides in the feed material. The bed material passes sequentially from
zone to zone via openings 58a,b in the baffles 30a,b separating the
various zones 34a-c. The conversion rate of sulfides to oxides in the
first zone 34a is higher than that in the second zone 34b, and the
conversion rate in the second zone 34b is higher than that in the third
zone 34c. The roaster off-gas 50 from the various zones 34a-c moves from
the bed 46 to the upper portion 62 of the chamber 14 and then passes to
the gas/particulate separation device 110. A portion of the entrained
particles from the bed 46 fall from the gas as the velocity of the gas
slows in response to the increased cross-sectional area of flow in the
upper portion of the chamber 14. Any remaining entrained particles are
removed by the gas/particulate separation device 110 and returned to the
bed via the underflow pipe 114. The treated off-gas 52 leaves the
gas/particulate separation device 110 for further treatment.
The preceding steps are repeated zone by zone as the feed material moves
from the first zone 34a to the second zone 34b and from the second zone
34b to the third zone 34c and so on. In the last zone 34c, the bed
material, which has now been fully oxidized to form the product, is
removed from the roaster via discharge port 22.
As noted above, the operating conditions in each zone can be different from
the operating conditions in other zones due to differences in the sulfide
contents of the bed particles in each zone. By way of example, the bed
particles in the first zone 34a commonly have a higher sulfide content
than the bed particles in the second zone 34b, and the bed particles in
the second zone 34b have a higher sulfide content than the bed particles
in the third zone 34c and so on. The sulfide content of the partially
oxidized material leaving the various zones 34a,b via openings 58a,b will
depend upon the reactor design and operating conditions.
The fluidizing gas velocity can differ from zone to zone. The superficial
velocity of the fluidizing gas in the initial (i.e., upstream) zones
preferably ranges from about 10 to about 80 cm/sec and more preferably
from about 20 to about 60 cm/sec to supply the desired amount of oxygen to
the bed. During the final stages of roasting, the rate of sulfur removal
drops and the continued use of a high fluidizing gas flow rate is
undesirable, since it would reduce the sulfur dioxide concentration in the
exit gas and remove heat from the bed where there is no significant heat
generation. Accordingly, a higher fluidizing gas flow rate can be used in
the initial zones than is used in the final (i.e., downstream) zones. The
fluidizing gas superficial velocity in the later zones should be
maintained low, preferably ranging from about 10 to about 40 cm/sec and
more preferably from about 10 to about 30 cm/sec. This can be accomplished
since air flow to each section of the plenum chamber can be controlled
separately.
Under the operating conditions described above, the roasting reaction
typically requires at least about 2 hours but no more than about 8 hours
and more typically is completed in about four to five hours. Depending
upon the gas flow rate and oxygen content used during the initial stages
of roasting a majority of the sulfur can be removed in the first hour. The
time required for the final zones is mainly dependent upon the final
desired sulfur content of the product.
FIGS. 3 and 4 depict a further embodiment of a fluidized bed roaster
according to the present invention. The roaster 150 is substantially
identical to the roaster 10 of FIGS. 1 and 2 with the exception that the
distributor plate 154 and bottom panel 158 of the chamber 14 are inclined
relative to the horizontal. The slope of the distributor plate 154
provides for variable residence times of the bed material (and fluidizing
gas) in each zone by causing the depth of the bed 46 to vary substantially
continuously along the length of the distributor plate 154. Thus, the bed
depth at the input end of the chamber 14 is less than the bed depth at the
discharge end of the chamber 14. Likewise, the bed depth in the first zone
34a is less than the bed depth in the second zone 34b and the bed depth in
the second zone 34b is less than the bed depth in the third zone 34c. In
this manner, the sloping distributor plate 154 causes a residence time of
the bed particles (and the retention time of the fluidizing gas) to
increase as the particles move from zone to zone towards the discharge end
of the roaster. The residence time of the particles in the first zone 34a
(and retention time of the fluidizing gas) is less than the residence time
of the particles in the second zone 34b (and retention time of the
fluidizing gas) and the residence time of the particles in the second zone
34b (and retention time of the fluidizing gas) is less than the residence
time of the particles in the third zone 34c (and retention time of the
fluidizing gas).
Alternatively, the distributor plate 154 can provide variations in
residence time by being stepped rather than sloped. In this design, the
distributor plate 154 can be stepped zone-by-zone to provide for
increasing bed depth as particles move from the feed end to the discharge
end of the chamber. In either case, the greater gas retention time and
residence time during the later stage of roasting when the reaction is
slowest is highly advantageous. It maximizes the utilization of the oxygen
in the air and the plant capacity rate, per square foot of hearth area,
while producing a low-sulfur product. As will be appreciated, a fluidized
bed reactor with level (no slope or steps) but longer hearth can also be
used to provide extra residence time to complete the roasting process.
EXAMPLES
The following examples are provided for illustrative purposes. They are not
intended to limit the scope of the invention as described above and in the
claims.
Example 1
Two byproduct molybdenite concentrates, one high grade containing about 92
percent MoS.sub.2 and the other low grade containing about 80 percent
MoS.sub.2, were blended in 4:1 weight proportion. This blend was used as
the feedstock in both the bench-scale batch tests and pilot-scale
continuous tests. The chemical and physical characteristics of this
feedstock are summarized below:
Chemical Analyses (weight percent)
Molybdenum 53
Sulfur 37
Copper 1
Iron 1.5
Calcium 0.3
Rhenium 500 ppm
Insoluble 7
Particle Density 4.71 g/cc
Bulk Density 1.4 g/cc
Particle Size Distribution
Diameter (Microns) Cumulative Percent Finer
80 95.4
60 89.8
40 75.5
20 43.1
10 19.2
5 8.0
Example 2
Batch Fluidized-Bed Roasting
A byproduct molybdenite concentrate of the composition given in Example 1
and containing approximately 0.7% residual flotation oil and 0.6% moisture
was roasted in a vibratory bench scale cylindrical fluidized-bed reactor
in a batch mode. Prior to roasting, 1,000 grams of the concentrate was
charged to the reactor and the reactor was sealed. To initiate the
roasting reaction, fluidizing air was passed through the reactor at 19.5
lpm (standard liters per minute) while the external walls were heated to
between 500.degree. C. and 600.degree. C. As the temperature of the
fluidized bed of molybdenum sulfide concentrate passed (nominally)
420.degree. C., the rate of temperature rise increased indicating an
exotherm within the reactor. As the temperature within the fluidized bed
passed (nominally) 480.degree. C., the exotherm caused a rapid temperature
increase and immediate temperature control was established by passing
cooling water through a coil located within the fluidized bed. At that
time, the auxiliary external furnace was turned off to prevent overheating
of the bed material.
Throughout the roasting reaction, the temperature of the fluidized bed was
controlled between 550.degree. C. and 560.degree. C. during which the bed
was gradually converted from molybdenum disulfide to molybdenum trioxide.
After approximately 140 minutes, the reaction exotherm subsided and the
cooling water was turned off while the auxiliary external furnace was
turned back on to hold the fluidized-bed temperature above 550.degree. C.
It can be appreciated that although the exotherm had subsided, it was
necessary to hold the bed at temperature for an additional 60 minutes to
allow removal of the last remaining sulfur.
Analysis of the reactor off-gas during the roasting reaction showed a sharp
increase in the sulfur dioxide concentration to approximately 12.5% when
the reaction exotherm started and a gradual decrease to 10% at the point
when the exotherm subsided. At this point, the sulfur dioxide
concentration dropped to roughly 5% and then steadily decreased to zero
during the 60 minute hold period, thus indicating all of the residual
sulfide had been fully roasted to oxide.
Example 3
Batch Fluidized-Bed Roasting Utilizing Oxygen Enriched Air
Another batch-wise fluidization test was conducted as described in the
previous example with the exception that 1,200 grams of molybdenite
concentrate was charged to the reactor and once the exotherm was
established, oxygen was mixed with the fluidizing air to achieve a
fluidizing gas containing 37% oxygen. During the exotherm, which lasted in
this case for 168 minutes, the sulfur dioxide concentration in the off-gas
rose to a high of 26% and remained fairly constant until the exotherm
subsided. The concentration then dropped steadily to zero over the next 62
minutes until the test was terminated.
The use of oxygen enrichment of the fluidizing air will result in increased
commercial production rates for a fixed-size fluidized-bed roaster.
Example 4
Continuous Fluidized-Bed Roasting
A byproduct molybdenite concentrate containing approximately 90% molybdenum
disulfide, 1.5% iron, 1.0% copper, 0.3 % Ca, 0.7% residual flotation oil
and 0.6 % moisture was roasted in a vibratory pilot-scale three-zone
continuous fluidized-bed reactor of the type described hereinbefore by
continuously feeding the concentrate at a rate of 8 lb/hr while passing
fluidizing air through the reactor at nominally 10 scfm. The temperature
of the fluidized bed, comprised of a mixture of molybdenum sulfide plus
molybdenum oxide, was controlled between 500.degree. C. and 560.degree. C.
by passing cooling water through coils located within the fluidized bed.
Molybdic oxide product was removed from the reactor at approximately 7
lb/hr after a nominal 3-hour residence time via a water-cooled
overflow-type discharge port, The rate of product discharge was
proportional to the feed rate taking into account the stoichiometric
weight change due to the conversion of molybdenum sulfide to molybdenum
oxide. The exhaust gas leaving the reactor was continuously analyzed and
found to contain 6-8% sulfur dioxide. Upon leaving the reactor, the
off-gas, which also contained low levels of volatile rhenium oxide and a
significant quantity of carry-over fines (small particle size bed
material) was passed through primary and secondary hot cyclones which
returned greater than 99% of the fines back to the reactor.
The reactor off-gas was then passed through a wet vortex-type scrubber
which removed residual particulate solids and the water-soluble volatile
rhenium oxide. Prior to discharge to the atmosphere, the sulfur dioxide in
the off-gas was removed by contact with a caustic solution in a
packed-tower scrubber.
While various embodiments of the present invention have been described in
detail, it is apparent that modifications and adaptations of those
embodiments will occur to those skilled in the art. However, it is to be
expressly understood that such modifications and adaptations are within
the scope of the present invention, as set forth in the following claims.
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